The present disclosure relates to the field of image processing, and more specifically to digital image processing. It finds particular application in conjunction with an apparatus and/or method for improving image contrast and will be described with reference thereto. However, those skilled in the art will appreciate that it is also amenable to other like applications.
A common reason for not using or liking photographs or other digital images is “poor lighting”. This usually means that an image has regions that are too dark or too light. Often such defects can only be improved or enhanced by local image processing and not by a single global histogram transformation. Additionally, more digital photographs with 14-bits per channel, or high dynamic range (HDR) images are being produced. These images are best reduced for 8-bit printing by local dynamic range compression.
HDR images can easily contain four orders of magnitude in light intensity, while displays and prints are limited to two to three orders of magnitude in emissive or reflective intensity. Capture devices that handle the HDR of light intensities are being developed. Slide film, for example, has a substantially higher dynamic range than print or most display devices, giving a source of relatively high dynamic range images. Given such an image, the challenge is to reproduce and/or output the image without losing the details in the highlights and shadows.
Many techniques for local contrast enhancement and dynamic range compression have been proposed in the prior art, however, most of these suffer from artifacts. Some techniques known in the art, such as bilateral filtering, have been shown to avoid these artifacts but at some computational expense. Furthermore, it has been shown that applying local contrast enhancement indiscriminately may actually “damage” or lessen the appeal of the enhanced image to users and/or observers. For example, in some tests, users may note that “artistic shadows” are appealing and that “grey highlights in the hair” and “reduction in specular appearance of shiny surfaces” are not. In such cases, a user and/or observer will prefer the results of a global contrast enhancement.